GaN-based light emitting diodes (LEDs) typically comprise an insulating, semiconducting or conducting substrate such as sapphire or SiC on which a plurality of GaN-based epitaxial layers are deposited. The epitaxial layers comprise an active region having a p-n junction that emits light when energized. A typical LED is mounted substrate side down onto a submount, also called a package or lead frame (hereinafter referred to as a “submount”). FIG. 1 schematically illustrates a conventional LED having an n-type SiC substrate 10, an active region 12 comprising an n-GaN-based layer 14 and a p-GaN-based layer 16 grown on the substrate and patterned into a mesa. A metal p-electrode 18 is deposited on and electrically coupled to the p-GaN layer 16 and a wire bond connection 28 is made to a bond pad 20 on the p-electrode 18. An n-electrode 22 on and electrically coupled to the conductive substrate is attached to metallic submount 24 using a conductive epoxy 26. In the conventional process, the conductive epoxy 26 (usually silver epoxy) is deposited on the submount and the LED is pressed into the epoxy 26. The epoxy is then heat cured which causes it to harden, providing a stable and electrically conductive mount for the LED chip. Light generated in the active region 12 is directed up and out of the device. However, a substantial amount of the generated light may be transmitted into the substrate and partially absorbed by the epoxy 26.
Flip-chip mounting of LEDs involves mounting the LED onto the submount substrate side up. Light is then extracted and emitted through the transparent substrate. Flip chip mounting may be an especially desirable technique for mounting SiC-based LEDs. Since SiC has a higher index of refraction than GaN, light generated in the active region does not internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip chip mounting of SiC-based LEDs may offer improved light extraction when employing certain chip-shaping techniques known in the art. Flip chip packaging of SiC LEDs may have other benefits as well, such as improved heat extraction/dissipation, which may be desirable depending on the particular application for the chip.
One problem with flip-chip mounting is illustrated in FIG. 2. Namely, when a chip is flip-chip mounted on a conductive submount or package conventional techniques may not be possible. Conventionally, a conductive die attach material 26, such as silver epoxy, is deposited on the chip and/or on the submount 24, and the chip is pressed onto the submount 24. This can cause the viscous conductive die attach material 26 to squeeze out and make contact with the n-type layers 14 and 10 in the device, thereby forming a Schottky diode connection that shunts the p-n junction in the active region with predictably undesirable results. Thus, new techniques may be needed for flip-chip mounting of LEDs.